US published patent applications 2010/0174384 (“the '384 application”) and 2006/0249315, each of which is incorporated herein by reference, describe that the gait cycle for walking can be divided into five phases: controlled plantarflexion, controlled dorsiflexion (CD), powered plantarflexion (PP), early swing, and late swing, as depicted in FIG. 1.
The '384 application also discloses a number of embodiments of lower-extremity prosthetic and orthotic systems in which the reflex torque generation during PP is achieved via non-linear, positive feedback between the series elastic element (SEE) motor torque and ankle torque. More specifically, the reflex action involves behaving like a non-linear spring during CD and like a torque source during PP. This reflex action can be implemented by driving the motor using the following equation:Motor Torque=pff×(normalized_Torque)n  Eq. 1Where, pff is the power control gain tuned for high walking speed; normalized_Torque is the ankle torque, ΓA, normalized by a torque, Γ0, (strongly related to users' weight); n is the power exponent, typically in the range of between 3 and 5 for level-ground walking. Note that pff has units of N-m, and the value of pff controls the magnitude of the level of the torque reflex during fast walking. Once the desired motor torque is determined, the drive current can be computed based on the equation Motor Current=Motor Torque/kt, where kt is the motor torque constant. While using Equation 1 does provide good results, the results provided by the control approach described below are significantly better.